APPARATUS, METHOD AND CORRESPONDING COMPUTER PROGRAM FOR
GENERATING AN ERROR CONCEALMENT SIGNAL USING INDIVIDUAL
REPLACEMENT LPC REPRESENTATIONS FOR INDIVIDUAL CODEBOOK
INFORMATION
Specification
The present invention relates to audio coding and in particular to audio coding based on
LPC-like processing in the context of codebooks.
Perceptual audio coders often utilize linear predictive coding (LPC) in order to model the
human vocal tract and in order to reduce the amount of redundancy, which can be
modeled by the LPC parameters. The LPC residual, which is obtained by filtering the input
signal with the LPC filter, is further modeled and transmitted by representing it by one, two
or more codebooks (examples are: adaptive codebook, glottal pulse codebook, innovative
codebook, transition codebook, hybrid codebooks consisting of predictive and transform
parts).
In case of a frame loss, a segment of speech/audio data (typically 10 ms or 20 ms) is lost.
To make this loss as less audible as possible, various concealment techniques are
applied. These techniques usually consist of extrapolation of the past, received data. This
data may be: gains of codebooks, codebook vectors, parameters for modeling the
codebooks and LPC coefficients. In all concealment technology known from state-of-theart,
the set of LPC coefficients, which is used for the signal synthesis, is either repeated
(based on the last good set) or is extra-/interpolated.
ITU G.718 [1]: The LPC parameters (represented in the ISF domain) are extrapolated
during concealment. The extrapolation consists of two steps. First, a long term target SF
vector is calculated. This long term target ISF vector is a weighted mean (with the fixed
weighting factor&eta) of
• an ISF vector representing the average of the last three known ISF vectors, and
• an offline trained ISF vector, which represents a long-term average spectral shape.
This long term target SF vector is then interpolated with the last correctly received 1SF
vector once per frame using a time-varying factor alpha to allow a cross-fade from the last
received ISF vector to the long term target SF vector. The resulting ISF vector is
subsequently converted back to the LPC domain, in order to generate intermediate steps
(ISFs are transmitted every 20 s, interpolation generates a set of LPCs every 5 ms). The
LPCs are then used to synthesize the output signal by filtering the result of the sum of the
adaptive and the fixed codebook, which are amplified with the corresponding codebook
gains before addition. The fixed codebook contains noise during concealment. In case of
consecutive frame loss, the adaptive codebook is fed back without adding the fixed
codebook. Alternatively, the sum signal might be fed back, as done in AMR-WB [5].
In [2], a concealment scheme is described which utilizes two sets of LPC coefficients. One
set of LPC coefficients is derived based on the last good received frame, the other set of
LPC parameters is derived based on the first good received frame, but it is assumed that
the signal evolves in reverse direction (towards the past). Then prediction is performed in
two directions, one towards the future and one towards the past. Therefore, two
representations of the missing frame are generated. Finally, both signals are weighted
and averaged before being played out.
Fig. 8 shows an error concealment processing in accordance with the prior art. An
adaptive codebook 800 provides an adaptive codebook information to an amplifier 808
which applies a codebook gain gp to the information from the adaptive codebook 800. The
output of the amplifier 808 is connected to an input of a combiner 810. Furthermore, a
random noise generator 804 together with a fixed codebook 802 provides codebook
information to a further amplifier g . The amplifier g indicated at 806 applies the gain
factor gc, which is the fixed codebook gain, to the information provided by the fixed
codebook 802 together with the random noise generator 804. The output of the amplifier
806 is then additionally input into the combiner 810. The combiner 810 adds the result of
both codebooks amplified by the corresponding codebook gains to obtain a combination
signal which is then input into an LPC synthesis block 81 . The LPC synthesis block 814
is controlled by replacement representation which is generated as discussed before.
This prior art procedure has certain drawbacks
in order to cope with changing signal characteristics or in order to converge the LPC
envelope towards background noise like-properties, the LPC is changed during
concealment by extra/interpolation with some other LPC vectors. There is no possibility to
precisely control the energy during concealment. While there is the chance to control the
codebook gains of the various codebooks, the LPC will implicitly influence the overall level
or energy (even frequency dependent).
It might be envisioned to fade out to a distinct energy level (e.g. background noise level)
during burst frame loss. This is not possible with state-of-the-art technology, even by
controlling the codebook gains.
It is not possible to fade the noisy parts of the signal to background noise, while
maintaining the possibility to synthesize tonal parts with the same spectral property as
before the frame loss.
It is an object of the present invention to provide an improved concept for generating an
error concealment signal.
This object is achieved by an apparatus for generating an error concealment signal of
claim 1, a method of generating an error concealment signal of claim 14 or a computer
program of claim 15.
in an aspect of the present invention, the apparatus for generating an error concealment
signal comprises an LPC representation generator for generating a first replacement LPC
representation and a different, second replacement LPC representation. Furthermore, an
LPC synthesizer is provided for filtering a first codebook information using the first
replacement LPC representation to obtain a first replacement signal and for filtering a
second different codebook information using the second replacement LPC representation
to obtain a second replacement signal. The outputs of the LPC synthesizer are combined
by a replacement signal combiner combining the first replacement signal and the second
replacement signal to obtain the error concealment signal.
The first codebook is preferably an adaptive codebook for providing the first codebook
information and the second codebook as preferably a fixed codebook for providing the
second codebook information. In other words, the first codebook represents the tonal part
of the signal and the second or fixed codebook represents the noisy part of the signal and
therefore can be considered to be a noise codebook.
The first codebook information for the adaptive codebook is generated using a mean
value of ast good LPC representations, the last good representation and a fading value.
Furthermore, the LPC representation for the second or fixed codebook is generated using
the last good LPC representation fading value and a noise estimate. Depending on the
implementation, the noise estimate can be a fixed value, an offline trained value or it can
be adaptively derived from a signal preceding an error concealment situation.
Preferably, an LPC gain calculation for calculating an influence of a replacement LPC
representation is performed and this information is then used in order to perform a
compensation so that the power or loudness or, generally, an amplitude-related measure
of the synthesis signal is similar to the corresponding synthesis signal before the error
concealment operation.
In a further aspect, an apparatus for generating an error concealment signal comprises an
LPC representation generator for generating one or more replacement LPC
representations. Furthermore, the gain calculator is provided for calculating the gain
information from the LPC representation and a compensator is then additionally provided
for compensating a gain influence of the replacement LPC representation and this gain
compensation operates using the gain operation provided by the gain calculator. An LPC
synthesizer then filters a codebook information using the replacement LPC representation
to obtain the error concealment signal, wherein the compensator is configured for
weighting the codebook information before being synthesized by the LPC synthesizer or
for weighting the LPC synthesis output signal. Thus, any gain or power or amplituderelated
perceivable influence at the onset of an error concealment situation is reduced or
eliminated.
This compensation is not only useful for individual LPC representations as outlined in the
above aspect, but is also useful in the case of using only a single LPC replacement
representation together with a single LPC synthesizer.
The gain values are determined by calculating impulse responses of the last good LPC
representation and a replacement LPC representation and by particularly calculating an
rms value over the impulse response of the corresponding LPC representation over a
certain time which is between 3 and 8 s and is preferably 5 ms.
In an implementation, the actual gain value is determined by dividing a new r s value, i.e.
an rms value for a replacement LPC representation by an rms value of good LPC
representation.
Preferably, the single or several replacement LPC representations is/are calculated using
a background noise estimate which is preferably a background noise estimate derived
from the currently decoded signals in contrast to an offline trained vector simply
predetermined noise estimate.
In a further aspect, an apparatus for generating a signal comprises an LPC representation
generator for generating one or more replacement LPC representations, and an LPC
synthesizer for filtering a codebook information using the replacement LPC
representation. Additionally, a noise estimator for estimating a noise estimate during a
reception of good audio frames is provided, and this noise estimate depends on the good
audio frames. The representation generator is configured to use the noise estimate
estimated by the noise estimator in generating the replacement LPC representation.
Spectral representation of a past decoded signal is process to provide a noise spectral
representation or target representation. The noise spectral representation is converted
into a noise LPC representation and the noise LPC representation is preferably the same
kind of LPC representation as the replacement LPC representation. ISF vectors are
preferred for the specific LPC-related processing procedures.
Estimate is derived using a minimum statistics approach with optimal smoothing to a past
decoded signal. This spectral noise estimate is then converted into a time domain
representation. Then, a Levinson-Durbin recursion is performed using a first number of
samples of the time domain representation, where the number of samples is equal to an
LPC order. Then, the LPC coefficients are derived from the result of the Levinson-Durbin
recursion and this result is finally transformed in a vector. The aspect of using individual
LPC representations for individual codebooks, the aspect of using one or more LPC
representations with a gain compensation and the aspect of using a noise estimate in
generating one or more LPC representations, which estimate is not an offline-trained
vector but is a noise estimate derived from the past decoded signal are individually
useable for obtaining an improvement with respect to the prior art.
Additionally, these individual aspects can a so be combined with each other so that, for
example, the first aspect and the second aspect can be combined or the first aspect or the
third aspect can be combined or the second aspect and the third aspect can be combined
to each other to provide an even improved performance with respect to the prior art. Even
more preferably, al three aspects can be combined with each other to obtain
improvements over the prior art. Thus, even though the aspects are described by
separate figures all aspects can be applied in combination with each other, as can be
seen by referring to the enclosed figures and description.
Preferred embodiments of the present invention are subsequently described with respect
to the accompanying drawings, in which:
Fig. 1a illustrates an embodiment of the first aspect;
Fig. 1b illustrates a usage of an adaptive codebook;
Fig. 1c illustrates a usage of a fixed codebook in the case of a normal mode or a
concealment mode;
Fig. 1d flowchart for calculating the first LPC replacement
Fig. 1e illustrates a flowchart for calculating the second LPC replacement
representation;
Fig. 2 illustrates an overview over a decoder with error concealment controller
and noise estimator;
Fig. 3 illustrates a detailed representation of the synthesis filters;
Fig. 4 illustrates a preferred embodiment combining the first aspect and the
second aspect;
Fig. 5 illustrates a further embodiment combining the first and second aspects;
Fig 6 illustrates the embodiment combining the first and second aspects;
Fig. 7a illustrates an embodiment for performing a gain compensation.
Fig. 7b illustrates a flowchart for performing a gain compensation;
Fig. 8 illustrates a prior art error concealment signal generator;
Fig. 9 illustrates an embodiment in accordance with the second aspect with gain
compensation;
illustrates a further implementation of the embodiment of Fig. 9;
illustrates an embodiment of the third aspect using the noise estimator;
illustrates a preferred implementation for calculating the noise estimate;
illustrates a further preferred implementation for calculating the noise
estimate; and
illustrates the calculation of a single LPC replacement representation or
individual LPC replacement representations for individual codebooks using
a noise estimate and applying a fading operation.
Preferred embodiments of the present invention relate to controlling the level of the output
signal by means of the codebook gains independently of any gain change caused by an
extrapolated LPC and to control the LPC modeled spectral shape separately for each
codebook. For this purpose, separate LPCs are applied for each codebook and
compensation means are applied to compensate for any change of the LPC gain during
concealment.
Embodiments of the present invention as defined in the different aspects or in combined
aspects have the advantage of providing a high subjective quality of speech/audio in case
of one or more data packets not being correctly or not being received at all at the decoder
side.
Furthermore, the preferred embodiments compensate the gain differences between
subsequent LPCs during concealment, which might result from the LPC coefficients being
changed over time, and therefore unwanted level changes are avoided.
Furthermore, embodiments are advantageous in that during concealment two or more
sets of LPC coefficients are used to independently influence the spectral behavior of
voiced and unvoiced speech parts and also tonal and noise-like audio parts.
All aspects of the present invention provide an improved subjective audio quality.
According to one aspect of this invention, the energy is precisely controlled during the
interpolation. Any gain that is introduced by changing the LPC is compensated.
According to another aspect of this invention, individual LPC coefficient sets are utilized
for each of the codebook vectors. Each codebook vector is filtered by its corresponding
LPC and the individual filtered signals are just afterwards summed up to obtain the
synthesized output. In contrast, state-of-the-art technology first adds up all excitation
vectors (being generated from different codebooks) and just then feeds the sum to a
single LPC filter.
According to another aspect, a noise estimate is not used, for example as an offlinetrained
vector, but is actually derived from the past decoded frames so that, after a certain
amount of erroneous or missing packets/frames, a fade-out to the actual background
noise rather than any predetermined noise spectrum is obtained. This particularly results
in a feeling of acceptance at a user side, but to the fact that even when an error situation
occurs, the signal provided by the decoder after a certain number of frames is related to
the preceding signal. However, the signal provided by a decoder in the case of a certain
number of lost or erroneous frames is a signal completely unrelated to the signal provided
by the decoder before an error situation.
Applying gain compensation for the time-varying gain of the LPC allows the following
advantages:
It compensates any gain that is introduced by changing the LPC.
Hence, the level of the output signal can be controlled by the codebook gains of the
various codebooks. This allows for a pre-determined fade-out by eliminating any
unwanted influence by the interpolated LPC,
Using a separate set of LPC coefficients for each codebook used during concealment
allows the following advantages:
It creates the possibility to influence the spectral shape of tonal and noise like parts of the
signal separately.
It gives the chance to play out the voiced signal part almost unchanged (e.g. desired for
vowels), while the noise part may quickly be converging to background noise.
It gives the chance to conceal voiced parts, and fade out the voiced part with arbitrary
fading speed (e.g. fade out speed dependent from signal characteristics), while
simultaneously maintaining the background noise during concealment. State-of-the-art
codecs usually suffer from a very clean voiced concealment sound.
It provides means to fade to background noise during concealment smoothly, by fading
out the tonal parts without changing the spectral properties, and fading the noise iike parts
to the background spectral envelope.
Fig. 1a illustrates an apparatus for generating an error concealment signal 111. The
apparatus comprises an LPC representation generator 100 for generating a first
replacement representation and additionally for generating a second replacement LPC
representation. As outlined in Fig. 1a, the first replacement representation is input into an
LPC synthesizer 106 for filtering a first codebook information output by a first codebook
102 such as an adaptive codebook 102 to obtain a first replacement signal at the output of
block 106. Furthermore, the second replacement representation generated by the LPC
representation generator 100 is input into the LPC synthesizer for filtering a second
different codebook information provided by a second codebook 104 which is, for example,
a fixed codebook, to obtain a second replacement signal at the output of block 108. Both
replacement signals are then input into a replacement signal combiner 110 for combining
the first replacement signal and the second replacement signal to obtain the error
concealment signal 111. Both LPC synthesizers 106, 108 can be implemented in a single
LPC synthesizer block or can be implemented as separate LPC synthesizer filters. In
other implementations, both LPC synthesizer procedures can be implemented by two LPC
filters actually being implemented and operating in parallel. However, the LPC synthesis
can also be an LPC synthesis filter and a certain control so that the LPC synthesis filter
provides an output signal for the first codebook information and the first replacement
representation and then, subsequent to this first operation, the control provides the
second codebook information and the second replacement representation to the synthesis
filter to obtain the second replacement signal in a serial way. Other implementations for
the LPC synthesizer apart from a single or several synthesis blocks are clear for those
skilled in the art.
Typically, the LPC synthesis output signals are time domain signals and the replacement
signal combiner 110 performs a synthesis output signal combination by performing a
synchronized sample-by-sample addition. However, other combinations, such as a
weighted sample-by-sample addition or a frequency domain addition or any other signal
combination can be performed by the replacement signal combiner 0 as well.
Furthermore, the first codebook 102 is indicated as comprising an adaptive codebook and
the second codebook 104 is indicated as comprising a fixed codebook. However, the first
codebook and the second codebook can be any codebooks such as a predictive
codebook as the first codebook and a noise codebook as the second codebook. However,
other codebooks can be glottal pulse codebooks, innovative codebooks, transition
codebooks, hybrid codebooks consisting of predictive and transform parts, codebooks for
individual voice generators such as males/females/children or codebooks for different
sounds such as for animal sounds, etc.
Fig. 1b illustrates a representation of an adaptive codebook. The adaptive codebook is
provided with a feedback loop 120 and receives, as an input, a pitch lag 118. The pitch lag
can be a decoded pitch lag in the case of a good received frame/packet. However, if an
error situation is detected indicating an erroneous or missing frame/packet, then an error
concealment pitch lag 1 8 is provided by the decoder and input into the adaptive
codebook. The adaptive codebook 102 can be implemented as a memory storing the fed
back output values provided via the feedback line 1 0 and, depending on the applied pitch
lag 118, a certain amount of sampling values is output by the adaptive codebook.
Furthermore, Fig. 1c illustrates a fixed codebook 104. In the case of the normal mode, the
fixed codebook 104 receives a codebook index and, in response to the codebook index, a
certain codebook entry 14 is provided by the fixed codebook as codebook information.
However, if a concealment mode is determined, a codebook index is not available. Then,
a noise generator 1 2 provided within the fixed codebook 104 is activated which provides
a noise signal as the codebook information 116. Depending on the implementation, the
noise generator may provide a random codebook index. However, it is preferred that a
noise generator actually provides a noise signal rather than a random codebook index.
The noise generator 112 may be implemented as a certain hardware or software noise
generator or can be implemented as noise tables or a certain "additional" entry in the fixed
codebook which has a noise shape. Furthermore, combinations of the above procedures
are possible, i.e. a noise codebook entry together with a certain post-processing .
Fig. 1d illustrates a preferred procedure for calculating a first replacement LPC
representation in the case of an error. Step 130 illustrates the calculation of a mean value
of LPC representations of two or more last good frames. Three last good frames are
preferred. Thus, a mean value over the three last good frames is calculated in block 130
and provided to block 136. Furthermore, a stored last good frame LPC information is
provided in step 132 and additionally provided to the block 136. Furthermore, a fading
factor 134 is determined in block 134. Then, depending on the last good LPC information,
depending on the mean value of the LPC information of the last good frame and
depending on the fading factor of block 134, the first replacement representation 138 is
calculated.
For the state-of-the-art just one LPC is applied . For the newly proposed method, each
excitation vector, which is generated by either the adaptive or the fixed codebook, is
filtered by its own set of LPC coefficients. The derivation of the individual ISF vectors is as
follows:
Coefficient set A (for filtering the adaptive codebook) is determined by this formula:
isf = i (block 136)
= alphaA isf 2 + (1 - alpha) isf (block 36)
where alphaA is a time varying adaptive fading factor which may depend on signal
stability, signal class, etc. isf x are the SF coefficients, where x denotes the frame
number, relative to the end of the current frame: x = - 1 denotes the first lost ISF, x = -2 the
last good, x=-3 second last good and so on.
This leads to fading the LPC which is used for filtering the tonal part, starting from the last
correctly received frame towards the average LPC {averaged over three of the last good
20ms frames). The more frames get lost, the closer the ISF, which is used during
concealment, will be to this short term average ISF vector (isf).
Fig . 1e illustrates a preferred procedure for calculating the second replacement
representation. In block 140, a noise estimate is determined. Then, in block 142, a fading
factor is determined. Additionally, in block 144, the last good frame is LPC information
which has been stored before is provided. Then, in block 146, a second replacement
representation is calculated. Preferably, a coefficient set B (for filtering the fixed
codebook) is determined by this formula:
isfe = alphaB · isf 2 + (1 beta) isf 3 (block 146)
where isf cn- is the ISF coefficient set derived from a background noise estimate and
alphaB is the time-varying fading speed factor which preferably is signal dependent. The
target spectral shape is derived by tracing the past decoded signal in the FFT domain
(power spectrum), using a minimum statistics approach with optimal smoothing, similar to
[3]. This FFT estimate is converted to the LPC representation by calculating the auto
correlation by doing inverse FFT and then using Levinson-Durbin recursion to calculate
LPC coefficients using the first N samples of the inverse FFT, where N is the LPC order.
This LPC is then converted into the ISF domain to retrieve is . Alternatively - if such
tracing of the background spectral shape is not available - the target spectral shape might
also be derived based on any combination of an offline trained vector and the short-term
spectral mean, as it is done in G.7 18 for the common target spectral shape.
Preferably, the fading factors A and sB are determined depending on the decoded audio
signal, i.e. , depending on the decoded audio signal before the occurrence of an error. The
fading factor may depend on signal stability, signal class, etc. Thus, is the signal is
determined to be a quite noisy signal, then the fading factor is determined in such a way
that the fading factor decreases, from time to time, more quickly than compared to a
situation where a signal is quite tonal. I this situation, the fading factor decreases from
one time frame to next time frame by a reduced amount. This makes sure that the fading
out from the last good frame to the mean value of the last three good frames takes place
more quickly in the case of noisy signals compared to non-noisy or tonal signals, where
the fading out speed is reduced. Similar procedures can be performed for signal classes.
For voiced signals, a fading out can be performed slower than for unvoiced signals or for
music signals a certain fading speed can be reduced compared to further signal
characteristics and corresponding determinations of the fading factor can be applied.
As discussed in the context of Fig. 1e , a different fading factor aB can be calculated for the
second codebook information. Thus, the different codebook entries can be provided with a
different fading speed . Thus, a fading out to the noise estimate as f n can be set
differently from the fading speed from the last good frame SF representation to the mean
SF representation as outlined in block 38 of Fig. 1 .
Fig. 2 illustrates an overview of a preferred implementation. An input line receives, for
example, from a wireless input interface or a cable interface packets or frames of an audio
signal. The data on the input line 202 is provided to a decoder 204 and at the same time
to an error concealment controller 200. The error concealment controller determines
whether received packet or frames are erroneous or missing. If this is determined, the
error concealment controller inputs a control message to the decoder 204. n the Fig . 2
implementation, a "1" message on the control line CTRL signals that the decoder 204 is to
operate in the concealment mode. However, if the error concealment controller does not
find an error situation, then the control line CTRL carries a "0" message indicating a
normal decoding mode as indicated in table 2 10 of Fig. 2. The decoder 204 is additionally
connected to a noise estimator 206. During the normal decoding mode, the noise
estimator 206 receives the decoded audio signal via a feedback line 208 and determines
a noise estimate from the decoded signal. However, when the error concealment
controller indicates a change from the normal decoding mode to the concealment mode,
the noise estimator 206 provides the noise estimate to the decoder 204 so that the
decoder 204 can perform an error concealment as discussed in the preceding and the
next figures. Thus, the noise estimator 206 is additionally controlled by the control line
CTRL from the error concealment controller to switch, from the normal noise estimation
mode in the normal decoding mode to the noise estimate provision operation in the
concealment mode.
Fig. 4 illustrates a preferred embodiment of the present invention in the context of a
decoder, such as the decoder 204 of Fig. 2, having an adaptive codebook 102 and
additionally having a fixed codebook 104. In the normal decoding mode indicated by a
control line data "0" as discussed in the context of the table 210 in Fig. 2, the decoder
operates as illustrated in Fig. 8, when item 804 is neglected. Thus, the correctly received
packet comprises a fixed codebook index for controlling the fixed codebook 802, a fixed
codebook gain g for controlling amplifier 806 and an adaptive codebook gp in order to
control the amplifier 808. Furthermore, the adaptive codebook 800 is controlled by the
transmitted pitch lag and the switch 812 is connected so that the adaptive codebook
output is fed back into the input of the adaptive codebook. Furthermore, the coefficients
for the LPC synthesis filter 804 are derived from the transmitted data.
However, if an error concealment situation is detected by the error concealment controller
202 of Fig. 2, the error concealment procedure is initiated in which, in contrast to the
normal procedure, two synthesis filters 106, 108 are provided. Furthermore, the pitch lag
for the adaptive codebook 102 is generated by an error concealment device. Additionally,
the adaptive codebook gain gp and the fixed codebook gain gc are also synthesized by an
error concealment procedure as known in the art in order to correctly control the amplifiers
402, 404.
Furthermore, depending on the signal class, a controller 409 controls the switch 405 in
order to either feedback a combination of both codebook outputs (subsequent to the
application of the corresponding codebook gain) or to only feedback the adaptive
codebook output.
In accordance with an embodiment, the data for the LPC synthesis filter A 106 and the
data for the LPC synthesis filter B 108 is generated by the LPC representation generator
100 of Fig. 1a and additionally a gain correction is performed by the amplifiers 406, 408.
To this end, the gain compensation factors g and gB are calculated in order to correctly
drive the amplifiers 408, 406 so that any gain influence generated by the LPC
representation is stopped. Finally, the output of the LPC synthesis filters A, B indicated by
06 and 108 are combined by the combiner 110, so that the error concealment signal is
obtained.
Subsequently, the switching from the normal mode to the concealment mode on one hand
and from the concealment mode back to the normal mode is discussed.
The transition from one common to several separate LPCs when switching from clean
channel decoding to concealment does not cause any discontinuities, as the memory
state of the last good LPC may be used to initialize each AR or MA memory of the
separate LPCs When doing so, a smooth transition from the last good to the first lost
frame is ensured,
When switching from concealment to clean channel decoding (recovery phase), the
approach of the separate LPCs introduces the challenge to correctly update the internal
memory state of the single LPC filter during clean-channel decoding (usually AR (autoregressive)
models are used). Just using the AR memory of one LPC or an averaged AR
memory would lead to discontinuities at the frame border between the last lost and the
first good frame. In the following a method is described to overcome deal with this
challenge:
A small portion of all excitation vectors (suggestion: 5ms) is added at the end of any
concealed frame. This summed excitation vector may then be fed to the LPC which would
be used for recovery. This is shown in Fig. 5. Depending on the implementation it is also
possible to sum up the excitation vectors after the LPC gain compensation.
It is advisable to start at frame end minus 5ms, setting the LPC AR memory to zero,
derive the LPC synthesis by using any of the individual LPC coefficient sets and save the
memory state at the very end of the concealed frame. If the next frame is correctly
received, this memory state may then be used for recovery (meaning: used for initializing
the start-of-frame LPC memory), otherwise it is discarded. This memory has to be
additionally introduced; it must be handled separately from any of the used LPC AR
memories of the concealment used during concealment.
Another solution for recovery is to use the method LPCO, known from USAC [4]
Subsequently, Fig. 5 is discussed in more detail. Generally, the adaptive codebook 02
can be termed to be a predictive codebook as indicated in Fig. 5 or can be replaced by a
predictive codebook. Furthermore, the fixed codebook 104 can be replaced or
implemented as the noise codebook 04. The codebook gains gp and g , in order to
correctly drive the amplifiers 402, 404 are transmitted, in the normal mode, in the input
data or can be synthesized by an error concealment procedure in the error concealment
case. Furthermore, a third codebook 412, which can be any other codebook, is used
which additionally has a associated codebook gain g as indicated by amplifier 414. In an
embodiment, an additional LPC synthesis by a separate filter controlled by an LPC
replacement representation for the other codebook is implemented in block 416.
Furthermore, a gain correction g is performed in a similar way as discussed in the context
of g and gB, as outlined.
Furthermore, the additional recovery LPC synthesizer X indicated at 418 is shown which
receives, as an input, a sum of at least a small portion of all excitation vectors such as
5 ms. This excitation vector is input into the LPC synthesizer X 418 memory states of the
LPC synthesis filter X.
Then, when a switchback from the concealment mode to the normal mode occurs, the
single LPC synthesis filter is controlled by copying the internal memory states of the LPC
synthesis filter X into this single normal operating filter and additionally the coefficients of
the filter are set by the correctly transmitted LPC representation.
Fig. 3 illustrates a further, more detailed implementation of the LPC synthesizer having
two LPC synthesis filters 106, 108. Each filter is, for example, an FIR filter or an IIR filter
having filter taps 304, 306 and filter-internal memories 304, 308. The filter taps 302, 306
are controlled by the corresponding LPC representation correctly transmitted or the
corresponding replacement LPC representation generated by the LPC representation
generator such as 100 of Fig. 1a. Furthermore, a memory initializer 320 is provided. The
memory initializer 320 receives the last good LPC representation and, when switch over to
the error concealment mode is performed, the memory initializer 320 provides the memory
states of the single LPC synthesis filter to the filter-internal memories 304, 308. In
particular, the memory initializer receives, instead of the last good LPC representation or
in addition to the last good LPC representation, the last good memory states, i.e. the
internal memory states of the single LPC filter in the processing, and particularly after the
processing of the last good frame/packet.
Additionally, as already discussed in the context of Fig. 5 , the memory initializer 320 can
also be configured to perform the memory initialization procedure for a recovery from an
error concealment situation to the normal non-erroneous operating mode. To this end, the
mernory initializer 320 or a separate future LPC memory initializer is configured for
initializing a single LPC filter in the case o a recovery from an erroneous or lost frame to a
good frame. The LPC memory initializer is configured for feeding at least a portion of a
combined first codebook information and second codebook information or at least a
portion of a combined weighted first codebook information or a weighted second
codebook information into a separate LPC filter such as LPC filter 418 of Fig. 5.
Additionally, the LPC memory initializer is configured for saving memory states obtained
by processing the fed in values. Then, when a subsequent frame or packet is a good
frame or packet, the single LPC filter 814 of Fig. 8 for the normal mode is initialized using
the saved memory states, i.e. the states from filter 418. Furthermore, as outlined in Fig. 5 ,
the filter coefficients for the filter can be either the coefficient for LPC synthesis filter 106
or LPC synthesis filter 08 or LPC synthesis filter 416 or a weighted or unweighted
combination of those coefficients.
Fig. 6 illustrates a further implementation with gain compensation. To this end, the
apparatus for generating an error concealment signal comprises a gain calculator 600 and
a compensator 406, 408, which has already been discussed in the context of Fig. 4 (406,
408) and Fig. 5 (406, 408, 409). In particular, the LPC representation calculator 100
outputs the first replacement LPC representation and the second replacement LPC
representation to a gain calculator 600. The gain calculator then calculates a first gain
information for the first replacement LPC representation and the second gain information
for the second LPC replacement representation and provides this data to the compensator
406, 408, which receives, in addition to the first and second codebook information, as
outlined in Fig. 4 or Fig. 5, the LPC of the last good frame/packet/block. Then, the
compensator outputs the compensated signal. The input into the compensator can either
be an output of amplifiers 402, 404, an output of the codebooks 102, 104 or an output of
the synthesis blocks 106, 108 in the embodiment of Fig. 4.
Compensator 406, 408 partly or fully compensates a gain influence of the first
replacement LPC in the first gain information and compensates a gain influence of the
second replacement LPC representation using the second gain information.
In an embodiment, the calculator 600 is configured to calculate a last good power
information related to a last good LPC representation before a start of the error
concealment. Furthermore, the gain calculator 600 calculates a first power information for
the first replacement LPC representation, a second power information for the second LPC
representation, the first gain value using the last good power information and the first
power information, and a second gain value using the last good power information and the
second power information. Then, the compensation is performed in the compensator 406,
408 using the first gain value and using the second gain value. Depending on the
information, however, the calculation of the last good power information can also be
performed, as illustrated in the Fig 6 embodiment, by the compensator directly. However,
due to the fact that the calculation of the last good power information is basically
performed in the same way as the first gain value for the first replacement representation
and the second gain value for the second replacement LPC representation, it is preferred
to perform the calculation of all gain values in the gain calculator 600 as illustrated by the
input 601.
In particular, the gain calculator 600 is configured to calculate from the last good LPC
representation or the first and second LPC replacement representations an impulse
response and to then calculate an rms (root mean square) value from the impulse
response to obtain the correspondent power information in the gain compensation, each
excitation vector is - after being gained by the corresponding codebook gain - again
amplified by the gains: g or gB . These gains are determined by calculating the impulse
response of the currently used LPC and then calculating the rms:
The result is then compared to the rms of the last correctly received LPC and the quotient
is used as gain factor in order to compensate for energy increase/loss of LPC
interpolation:
rms old
9 = rms-new
This procedure can be seen as a kind of normalization. It compensates the gain, which is
caused by LPC interpolation.
Subsequently, Figs. 7a and 7b are discussed in more detail to illustrate the apparatus for
generating an error concealment signal or the gain calculator 600 or the compensator
406, 408 calculates the last good power information as indicated at 700 in Fig. 7a.
Furthermore, the gain calculator 600 calculates the first and second power information for
the first and second LPC replacement representation as indicated at 702. Then, as
illustrated by 704, the first and the second gain values are calculated preferably by the
gain calculator 600. Then, the codebook information or the weighted codebook
information or the LPC synthesis output is compensated using these gain values as
illustrated at 706 This compensation is preferably done by the amplifiers 408, 408.
To this end, several steps are performed in an preferred embodiment as illustrated in Fig.
7b. In step 710, an LPC representation, such as the first or second replacement LPC
representation or the last good LPC representation is provided. In step 7 the codebook
gains are applied to the codebook information/output as indicated by block 402, 404.
Furthermore, in step 716, impulse responses are calculated from the corresponding LPC
representations. Then, in step 718, an rms value is calculated for each impulse response
and in block 720 the corresponding gain is calculated using an old rms value and a new
rms value and this calculation is preferably done by dividing the old rms value by the new
rms value. Finally, the result of block 720 is used to compensate the result of step 712 in
order to finally obtained the compensated results as indicated at step 714.
Subsequently, a further aspect is discussed, i.e. an implementation for an apparatus for
generating an error concealment signal which ha the LPC representation generator 100
generating only a single replacement LPC representation, such as for the situation
illustrated in Fig. 8. In contrast to Fig. 8 , however, the embodiment illustrating a further
aspect in Fig. 9 comprises the gain calculator 600 and the compensator 406, 408. Thus,
any gain influence by the replacement LPC representation generated by the LPC
representation generator is compensated for. In particular, this gain compensation can be
performed on the input side of the LPC synthesizer as illustrated in Fig. 9 by compensator
406, 408n or can be alternatively performed to the output of the LPC synthesizer as
illustrated by the compensator 900 in order to finally obtain the error concealment signal.
Thus, the compensator 406, 408, 900 is configured for weighting the codebook
information or an LPC synthesis output signal provided by the LPC synthesizer 106, 108.
The other procedures for the LPC representation generator, the gain calculator, the
compensator and the LPC synthesizer can be performed in the same way as discussed in
the context of Figs. 1a to 8 .
As has been outlined in the context of Fig. 4, the amplifier 402 and the amplifier 406
perform two weighting operations in series to each other, particularly in the case where
not the sum of the multiplier output 402, 404 is fed back into the adaptive codebook, but
where only the adaptive codebook output is fed back, i.e. when the switch 405 is in the
illustrated position or the amplifier 404 and the amplifier 408 perform two weighting
operations in series. In an embodiment, illustrated in Fig. 10, these two weighting
operations can be performed in a single operation. To this end, the gain calculator 600
provides its output g or g to a single value calculator 1002. Furthermore, a codebook
gain generator 1000 is implemented in order to generate a concealment codebook gain as
known in the art. The single value calculator 1002 then preferably calculators a product
between gp and g in order to obtain the single value. Furthermore, for the second branch,
the single value calculator 1002 calculates a product between g or gB in order to provide
the single value for the lower branch in Fig. 4 . A further procedure can be performed for
the third branch having amplifiers 414, 409 of Fig. 5.
Then a manipulator 1004 is provided which together performs the operations of for
example amplifiers 402, 406 to the codebook information of a single codebook or to the
codebook information of two or more codebooks in order to finally obtain a manipulated
signal such as a codebook signal or a concealment signal, depending on whether the
manipulator 1004 is located before the LPC synthesizer in Fig. 9 or subsequent to the
LPC synthesizer of Fig. 9 . Fig. 11 illustrates a third aspect, in which the LPC
representation generator 100, the LPC synthesizer 106, 08 and the additional noise
estimator 206, which has already been discussed in the context of Fig. 2, are provided.
The LPC synthesizer 106, 108 receives codebook information and a replacement LPC
representation. The LPC representation is generated by the LPC representation generator
using the noise estimate from the noise estimator 206, and the noise estimator 206
operates by determining the noise estimate from the last good frames. Thus, the noise
estimate depends on the last good audio frames and the noise estimate is estimated
during a reception of good audio frames, i.e. in the normal decoding mode indicated by "0"
on the control line of Fig. 2 and this noise estimate generated during the normal decoding
mode is then applied in the concealment mode as illustrated by the connection of blocks
206 and 204 i Fig. 2 .
The noise estimator is configured to process a spectral representation of a past decoded
signal to provide a noise spectral representation and to convert the noise spectral
representation into a noise LPC representation, where the noise LPC representation is the
same kind of an LPC representation as the replacement LPC representation. Thus, when
the replacement LPC representation is in the ISF-domain representation or an ISF vector,
then the noise LPC representation additionally is an ISF vector or ISF representation.
Furthermore, the noise estimator 206 is configured to apply a minimum statistics approach
with optimal smoothing to a past decoded signal to derive the noise estimate. For this
procedure, t is preferred to perform the procedure illustrated in [3]. However, other noise
estimation procedures relying on, for example, suppression of tonal parts compared to
non-tonal parts in a spectrum in order to filter out the background noise or noise in an
audio signal can be applied as well for obtaining the target spectral shape or noise
spectral estimate.
Thus, in one embodiment, a spectral noise estimate is derived from a past decoded signal
and the spectral noise estimate is then converted into an LPC representation and then
into an ISF domain to obtain the final noise estimate or target spectral shape.
Fig. 12a illustrates a preferred embodiment. In step 1200, the past decoded signal is
obtained, as for example illustrated in Fig. 2 by the feedback loop 208. In step 1202, a
spectral representation, such as a Fast Fourier transform (FFT) representation is
calculated. Then, in step 1204 a target spectral shape is derived such as by the minimum
statistics approach with optimal smoothing or by any other noise estimator processing.
Then, the target spectral shape is converted into an LPC representation as indicated by
block 1206 and finally the LPC representation is converted to an ISF factor as outlined by
block 1208 in order to finally obtain the target spectral shape in the ISF domain which can
then be directly used by the LPC representation generator for generating a replacement
LPC representation. In the equations of this application, the target spectral shape in the
ISF domain is indicated as "ISFcr,g" .
In a preferred embodiment illustrated in Fig. 12b, the target spectral shape is derived for
example by a minimum statistics approach and optimal smoothing. Then, in step 1212, a
time domain representation is calculated by applying an inverse FFT, for example, to the
target spectral shape. Then, LPC coefficients are calculated by using Levinson-Durbin
recursion. However, the LPC coefficients calculation of block 1214 can also be performed
by any other procedure apart from the mentioned Levinson-Durbin recursion. Then, in
step 1216, the final ISF factor is calculated to obtain the noise estimate ISF to be used
by the LPC representation generator 100.
Subsequently, Fig. 13 is discussed for illustrating the usage of the noise estimate in the
context of the calculation of a single LPC replacement representation 1308 for the
procedure, for example, illustrated in Fig. 8 or for calculating individual LPC
representations for individual codebooks as indicated by block 1310 for the embodiment
illustrated in Fig. 1.
In step 1300, a mean value of two or three last good frames is calculated. In step 1302,
the last good frame LPC representation is provided. Furthermore, in step 1304, a fading
factor is provided which can be controlled, for example, by a separate signal analyzer
which can be, for example, included in the error concealment controller 200 of Fig. 2.
Then, in step 1306, a noise estimate is calculated and the procedure in step 1306 can be
performed by any of the procedures illustrated in Figs. 12a, 12b.
In the context of calculating a single LPC replacement representation, the outputs of
blocks 1300, 1304, 1306 are provided to the calculator 1308. Then, a single replacement
LPC representation is calculated in such a way that subsequent to a certain number of
lost or missing or erroneous frames/packets, the fading over to the noise estimate LPC
representation is obtained.
However, individual LPC representations for an individual codebook, such as for the
adaptive codebook and the fixed codebook, are calculated as indicated at block 1310,
then the procedure as discussed before for calculating IS F (LPC A) on the hand and the
calculation of ISFB LPC B) is performed.
Although the present invention has been described in the context of block diagrams where
the blocks represent actual or logical hardware components, the present invention can
also be implemented by a computer-implemented method. In the latter case, the blocks
represent corresponding method steps where these steps stand for the functionalities
performed by corresponding logical or physical hardware blocks.
Although some aspects have been described in the context of an apparatus, it is clear that
these aspects also represent a description of the corresponding method, where a block or
device corresponds to a method step or a feature of a method step. Analogously, aspects
described in the context of a method step also represent a description of a corresponding
block or item or feature of a corresponding apparatus. Some or all of the method steps
may be executed by (or using) a hardware apparatus, like for example, a microprocessor,
a programmable computer or an electronic circuit. In some embodiments, some one or
more of the most important method steps may be executed by such an apparatus.
Depending on certain implementation requirements, embodiments of the invention can be
implemented in hardware or in software. The implementation can be performed using a
digital storage medium, for example a floppy disc, a DVD, a Blu-Ray, a CD, a ROM, a
PROM, and EPROM, an EEPROfvl or a FLASH memory, having electronically readable
control signals stored thereon, which cooperate (or are capable of cooperating) with a
programmable computer system such that the respective method is performed. Therefore,
the digital storage medium may be computer readable.
Some embodiments according to the invention comprise a data carrier having
electronically readable control signals, which are capable of cooperating with a
programmable computer system, such that one of the methods described herein is
performed.
Generally, embodiments of the present invention can be implemented as a computer
program product with a program code, the program code being operative for performing
one of the methods when the computer program product runs on a computer. The
program code may, for example, be stored on a machine readable carrier.
Other embodiments comprise the computer program for performing one of the methods
described herein, stored on a machine readable carrier.
In other words, an embodiment of the inventive method is, therefore, a computer program
having a program code for performing one of the methods described herein, when the
computer program runs on a computer.
A further embodiment of the inventive method is, therefore, a data carrier {or a nontransitory
storage medium such as a digital storage medium, or a computer-readable
medium) comprising, recorded thereon, the computer program for performing one of the
methods described herein. The data carrier, the digital storage medium or the recorded
medium are typically tangible and/or non-transitory.
A further embodiment of the invention method is, therefore, a data stream or a sequence
of signals representing the computer program for performing one of the methods
described herein. The data stream or the sequence of signals may, for example, be
configured to be transferred via a data communication connection, for example, via the
internet.
A further embodiment comprises a processing means, for example, a computer or a
programmable logic device, configured to, or adapted to, perform one of the methods
described herein.
A further embodiment comprises a computer having installed thereon the computer
program for performing one of the methods described herein.
A further embodiment according to the invention comprises an apparatus or a system
configured to transfer (for example, electronically or optically) a computer program for
performing one of the methods described herein to a receiver. The receiver may, for
example, be a computer, a mobile device, a memory device or the like. The apparatus or
system may, for example, comprise a file server for transferring the computer program to
the receiver .
In some embodiments, a programmable logic device (for example, a field programmable
gate array) may be used to perform some or all of the functionalities of the methods
described herein. In some embodiments, a field programmable gate array may cooperate
with a microprocessor in order to perform one of the methods described herein. Generally,
the methods are preferably performed by any hardware apparatus.
The above described embodiments are merely illustrative for the principles of the present
invention. It is understood that modifications and variations of the arrangements and the
details described herein will be apparent to others skilled in the art. It is the intent,
therefore, to be limited only by the scope of the impending patent claims and not by the
specific details presented by way of description and explanation of the embodiments
herein.
References
[1] ITU-T G.718 Recommendation, 2006
[2] Kazuhiro Kondo, Kiyoshi Nakagawa, „A Packet Loss Concealment Method Using
Recursive Linear Prediction" Department of Electrical Engineering, Yamagata University,
Japan.
[3] R. Martin, Noise Power Spectral Density Estimation Based on Optimal Smoothing and
Minimum Statistics, IEEE Transactions on speech and audio processing, vol. 9, no. 5, July
2001
[4] Ralf Geiger et. al., Patent application US201 1017301 A 1, Audio Encoder and
Decoder for Encoding and Decoding Frames of a Sampled Audio Signal
[5] 3GPP TS 26.190; Transcoding functions; - 3GPP technical specification

Claims
1. Apparatus for generating an error concealment signal, comprising:
an LPC (linear prediction coding) representation generator (100) for generating a
first replacement LPC representation and a different second replacement LPC
representation;
an LPC synthesizer (106) for filtering a first codebook information using the first
replacement representation to obtain a first replacement signal and for filtering a
different second codebook information using the second replacement LPC
representation to obtain a second replacement signal; and
a replacement signal combiner ( 1 10) for combining the first replacement signal and
the second replacement signal to obtain the error concealment signal ( 1 11).
2 . Apparatus of claim 1, further comprising:
an adaptive codebook (102) for providing the first codebook information; and
a fixed codebook (104) for providing the second codebook information.
3. Apparatus of claim 2,
wherein the fixed codebook (104) is configured to provide a noise signal ( 1 12) for
the error concealment, and
wherein the adaptive codebook (102) is configured for providing an adaptive
codebook content or an adaptive codebook content combined with earlier fixed
codebook content.
4. Apparatus o one of the preceding claims,
wherein the LPC representation generator (100) is configured to generate the first
replacement LPC representation using one or more two non-erroneous preceding
LPC representations, and
to generate the second replacement LPC representation using a noise estimate
and at least one non-erroneous preceding LPC representation.
Apparatus of claim 4,
wherein the LPC representation generator (100) is configured to generate the first
replacement LPC representation using a mean value of at ieast two last good
frames (130) and a weighted summation of the mean value and the last good
frame (136), wherein a first weighting factor of the weighted summation changes
over successive erroneous or lost frames,
wherein the LPC coefficient generator is configured to generate the second
replacement LPC representation only using a weighted summation (146) of a last
good frame ( 1 14) and the noise estimate (140), wherein a second weighting factor
of the weighted summation changes over successive erroneous or lost frames.
Apparatus of claim 4 or 5, further comprising;
a noise estimator (206) for estimating the noise estimate from one or more
preceding good frames (208).
Apparatus of one of the preceding claims,
further comprising an LPC memory initializer (320) for initializing, in case of an
error concealment situation (210), memory states (304, 308) of a first LPC filter
and second memory states (308) of second LPC filter values stored in
corresponding memory states of a single LPC filter used for a good frame
preceding an erroneous or ost frame.
Apparatus of one of the preceding claims, an LPC memory initializer for initializing
a single LPC filter in case of a recovery from an erroneous or lost frame to a good
frame, the LPC memory initializer being configured for:
feeding at least a portion of a combined first codebook information and second
codebook information or at least a portion of a combined weighted first codebook
information and a weighted second codebook information into an LPC filter (418),
saving memory states obtained by the feeding; and
initializing the single LPC filter using the saved memory states, when a subsequent
frame is a good frame.
9. Apparatus of one of the preceding claims,
further comprising a controller (409) for controlling a feedback into a first codebook
(102) providing the first codebook information, wherein the controller (409) is
configured to feed the first codebook information back into the first codebook or to
feed the combination of the first codebook information and the second codebook
information back into the first codebook.
0. Apparatus of one of the preceding claims, further comprising:
a gain calculator (600) for calculating a first gain information from the first
presentation a second gain information from the second replacement LPC
representation;
a compensator (406, 408) for compensating a gain influence of the first
replacement LPC information using the first gain information and for compensating
a gain influence of the second replacement LPC representation using the second
gain information.
1. Apparatus of claim 10,
wherein the gain calculator (600) is configured to calculate:
a last good power information (700) related to a last good LPC representation
before a start of the error concealment, a first power information (702) from the first
replacement LPC representation and a second power information from the second
replacement LPC representation,
a first gain value (704) using the last good power information and the first power
information and a second gain value using the last good power information and the
second power information, and
wherein the compensator (406, 408) is configured for compensating using the first
gain value and using the second gain value (706)
Apparatus of claim 1,
wherein the gain calculator (600) is configured to calculate an impulse response
(716) of an LPC representation and to calculate an RMS value (718) from the
impulse response to obtain a corresponding power information.
Apparatus of one of the preceding claims,
wherein the LPC representation generator is configured to generate SF vectors for
the replacement LPC representations.
A method of generating an error concealment signal, comprisingi
generating (100) a first replacement LPC representation and a different second
replacement LPC representation;
filtering (106) a first codebook information using the first replacement
representation to obtain a first replacement signal and filtering (108) a different
second codebook information using the second replacement LPC representation to
obtain a second replacement signal; and
combining ( 1 10) the first replacement signal and the second replacement signal to
obtain the error concealment signal ( 111) .
Computer program for performing, when running on a computer or a processor, the
method for generating an error concealment signal of claim 14.